The present invention relates to a process for the continuous manufacture of a dialkylaminoalkyl (meth)acrylate of formula (I): 
in which:
R1 is a hydrogen atom or a methyl radical;
A is a linear or branched C1-C5 alkylene radical; and
R2 and R3, which may be identical to or different from each other, each represent a C1-C4 alkyl radical,
by reaction in a stirred reactor between a compound of formula (II): 
in which A, R2 and R3 have the same meanings as above, and a compound of formula (III): 
in which:
R1 is as defined above; and
R4 is a linear alkyl group containing 1 or 2 carbon atoms,
in the presence of a tetraalkyl titanate as a transesterification catalyst and in the presence of at least one polymerization inhibitor, the compound (III)/R4OH azeotropic mixture being removed continuously during the reaction.
French patent No. 1,544,542 describes the preparation of dimethylaminoethyl acrylate from H2Cxe2x95x90CHCOOCH(CH3)2 or from H2Cxe2x95x90CHCOOCH2CH(CH3)2 and from alcohols in the presence of a catalyst such as n-propyl, isopropyl or isobutyl titanate and polybutyl titanate. This process has the drawback that the transesterification of titanates either with the light alcohol released during the reaction, or with the starting alcohol, causes the appearance of impurities in the reaction mixture and complicates the purification of the acrylic ester prepared.
In an attempt to overcome this problem, French patent No. 2,617,840 proposes a process for manufacturing compounds of formula (I) above, according to which ethyl (meth)acrylate is reacted, in the presence of at least one polymerization inhibitor, at 20-120xc2x0 C. and at a pressure equal to or less than atmospheric pressure, with an amino alcohol of formula (II) above in an ethyl (meth)acrylate/amino alcohol (II) molar ratio of between 1.5 and 5, in the presence of tetraethyl titanate, the ethyl (meth)-acrylate/ethanol azeotropic mixture being removed during the reaction, and the compound (I) obtained being separated out at the end of the reaction.
The problem currently arising is that of the production on an industrial scale, continuously and with high purity, of these compounds (I). Thus, for example, it is sought to obtain dimethylaminoethyl acrylate (DMAEA) containing less than 100 ppm of ethyl acrylate (EA) and less than 300 ppm of dimethylaminoethanol (DMAE).
European patent EP-B-0,160,427 describes a process for manufacturing DMAEA which can be represented schematically as follows:
(1) Preparation of the Catalyst 
(2) Synthesis of DMAEA by Exchange Reaction in the Presence of Methyl Acrylate 
compound B=compound A partially modified with CH3OH ligands from the methyl acrylate.
Compound B is then separated from the crude reaction mixture by a flash distillation, and the light phase obtained from this operation, containing the DMAEA and the excess unreacted methyl acrylate, is distilled to isolate the pure DMAEA.
(3) Regeneration of Compound A
Compound B+excess DMAExe2x86x92compound A+MeOH+DMAE
A flash distillation makes it possible to isolate compound A from the light phase containing the methanol and the DMAE. Compound A is recycled into step (2) and the light phase is distilled to separate the methanol from the DMAE, which is recycled into step (3).
This process, indicated in the abovementioned patent as being able to be carried out continuously, although no temperature or pressure data were provided for the abovementioned distillations, is still relatively complicated.
Brazilian patent No. PI 87/01337 describes a transesterification of a light (methyl) acrylate (without specifying that the heavy alcohol can be DMAE) continuously, according to which the reactor used is a tubular reactor in combination with an evaporator. The flow of vapour obtained in the evaporator is sent to a distillation column in order to separate:
the light acrylate/light alcohol (methanol) azeotrope at the top; and
the heavy acrylate and the heavy alcohol, with a small amount of light acrylate, at the bottom.
This last flow is sent to a purification column, from which a stream rich in heavy alcohol leaves, at the top, and is recycled into the reactor (topping). The titanate (catalyst) and the heavy acrylate leave at the bottom of this column. This flow is sent to a final column, from which the pure heavy acrylate leaves at the top and the catalyst leaves at the bottom.
This process does not make it possible, in the case of the synthesis of DMAEA, to obtain a product which satisfies the abovementioned specifications.
Brazilian patent No. PI 87/01338 describes, for the same reaction (still without specifically mentioning the amino alcohols as heavy alcohols), the use of a distillation column fed in the top part with the heavy alcohol and the catalyst (example: butyl titanate), and in the bottom part with the light acrylate. The flow leaving at the top is rich in light alcohol and contains light acrylate and heavy alcohol; the light acrylate is separated out and recycled into the column. The flow leaving at the bottom is rich in heavy acrylate (2-ethylhexyl acrylate in the example) and contains heavy alcohol and light acrylate.
It has now been discovered that by first carrying out a tailing operation (removal of the catalyst and the heavy products), followed by a topping operation and a final rectification, on a crude reaction mixture from the transesterification of alkyl (meth)acrylate (III) as defined above with an amino alcohol (II) as defined above, it is possible industrially to obtain a (meth)acrylate (I) of high purity.
A subject of the present invention is thus, firstly, a process for the continuous manufacture of a (meth)acrylate (I), as defined above, characterized in that the transesterification catalyst is chosen from tetrabutyl, tetraethyl and tetra(2-ethylhexyl) titanates, and in that the reaction is carried out in the stirred reactor at a temperature of 90-120xc2x0 C., after which the following steps are carried out:
the crude reaction mixture comprising the desired heavy ester (I) with, as light products, compound (II) and the unreacted light ester (III), and, as heavy products, the catalyst, the polymerization inhibitor(s) and heavy reaction products, is sent to a first distillation column (C1) under reduced pressure, and a distillation is carried out in the said first column (C1), which makes it possible to obtain:
at the top, a flow composed essentially of the heavy ester (I) and the light products, containing a small fraction of heavy products, but free or substantially free of catalyst; and
at the bottom, a flow of heavy products with a small fraction of heavy ester (I) and the catalyst; after which
the flow from the top of the first distillation column (C1) is sent to a second distillation column (C2) under reduced pressure, in which a distillation is carried out which makes it possible to obtain:
at the top, a flow of the light products with a small fraction of heavy ester (I); and
at the bottom, the heavy ester (I) containing traces of light products, heavy reaction products and the polymerization inhibitor(s); and
the flow from the bottom of the second distillation column (C2) is sent to a third distillation column (C3) under reduced pressure, in which a rectification is carried out which makes it possible to obtain:
at the top, the desired heavy ester (I); and
at the bottom, essentially the polymerization inhibitor(s).
In general, the reaction is carried out in the reactor (R) in an alkyl (meth)acrylate (III)/amino alcohol (II) molar ratio of between 1.1 and 3, preferably between 1.7 and 2.2; the catalyst is used in a proportion of from 10xe2x88x924 to 10xe2x88x922 mol per mole of amino alcohol (II), preferably in a proportion of from 1xc3x9710xe2x88x923 to 8xc3x9710xe2x88x923 mol per mole of amino alcohol (II);
the reaction is carried out in the reactor (R) at a pressure of between 650 millibar and atmospheric pressure.
Moreover, the stabilizer(s) is (are) chosen from phenothiazine, tert-butylcatechol, hydroquinone methyl ether, hydroquinone and mixtures thereof in all proportions, and it is (they are) used in a proportion of 100-5000 ppm relative to the reaction feedstock. A stabilizer such as phenothiazine can also be added into the column (C2).
As examples of amino alcohols (II), mention may be made of dimethylaminoethanol (DMAE), dimethylaminopropanol and diethylaminoethanol.
In accordance with preferred characteristics of the process according to the present invention,
the first distillation column (C1) is run at a pressure of 3.73xc3x97103-1.04xc3x97104 Pa (28-78 mm Hg) at a temperature at the bottom of 100-115xc2x0 C.;
the second distillation column (C2) is run at a pressure of 9.33xc3x97103-1.07xc3x97104 Pa (70-80 mm Hg) at a temperature at the bottom of 110-125xc2x0 C.;
the flow from the bottom of column (C1) is optionally recycled into the reactor (R) after purification by passage through a film evaporator, as is the flow from the top of column (C2); and
the rectification column (C3) is run at a pressure of 3.73xc3x97103-70.7xc3x97103 Pa (28-53 mm Hg) at 82-94xc2x0 C.
The examples which follow illustrate the present invention without, however, limiting its scope. The results of various operations have been collated in each of them. The percentages are given on a weight basis.